1. Field of the Invention
The present invention relates to an optical scanning apparatus, an image formation apparatus, and a phase modulation method. The present invention especially relates to
an optical scanning apparatus wherein a photo conductor is scanned by an optical beam,
an image formation apparatus that includes the optical scanning apparatus, and
a phase modulation method that modulates a phase of a liquid crystal device for deflecting an optical path of the optical beam that scans the photo conductor.
2. Description of the Related Art
Conventionally, image formation apparatuses include an optical scanning apparatus for forming a latent image on a photo conductor, wherein an optical beam irradiated by a luminous source is scanned by a deflecting unit such as a polygon mirror. With the conventional optical scanning apparatus, raising the processing rate is realized by raising the rotating speed of the polygon mirror.
Such image formation apparatuses are often used in on-demand printing systems that provide simple printing. There are increasing requirements for improvements in productivity and image quality. Then, in recent years and continuing, some of the image formation apparatuses are provided with an optical scanning apparatus that includes a multi-beam luminous source having two or more light emitting points (luminous channels) built into one package, where two or more optical beams are irradiated from corresponding light emitting points; or alternatively, two or more semiconductor lasers, each of which irradiates a single beam. In addition, the rotating speed of the polygon mirror is being raised.
Here, problems with the multi-beam luminous source are
that it is difficult to increase the number of channels for manufacturing reasons,
that it is difficult to remove thermal or electric crosstalk, and
that it is difficult to use a short wavelength. For these reasons, the multi-beam luminous source tends to be expensive. Accordingly, an image formation apparatus tends to be expensive if the multi-beam luminous source is used.
On the other hand, an optical scanning apparatus that uses two or more single beam semiconductor lasers as the luminous source has a problem in that a beam spot arrangement (beam pitch, and scanning line interval) at a scanned surface of a photo conductor fluctuates due to a change in the surrounding environment, and a secular change of the scanning apparatus. Then, a method of rectifying the problem (such as re-arrangement of the beam spot on the scanned surface) is proposed (for example, Patent Reference 1 and Patent Reference 2), wherein a liquid crystal device is provided in an optical path of the optical beam, and is driven by an electrical signal according to the fluctuation of the beam spot.
According to the method proposed by Patent Reference 1 and Patent Reference 2, a suitable voltage is applied to a liquid crystal layer of the liquid crystal device for deflecting the optical beam, and a refraction index of the optical beam at the liquid crystal layer is controlled by generating an electric potential distribution with a fixed gradient in the liquid crystal layer.
However, as a matter of fact, even if the electric potential distribution with the fixed gradient is generated in the liquid crystal layer, the gradient of the refraction index generated in the liquid crystal layer is not necessarily fixed. For example, the gradient of the refraction index takes a curved shape that can be better approximated by a high order polynomial. If the optical beam is made incident on the liquid crystal layer that has the refractive index distribution in the curved shape, the liquid crystal layer functions as a lens for the optical beam. That is, not only is the optical path of the optical beam deflected, but also wavefront aberration is generated, posing a problem. Further, when the temperature of the liquid crystal device changes the refractive index distribution is susceptible to change, causing a characteristic of the liquid crystal layer to fluctuate.    [Patent Reference 1] JPA 2003-302595    [Patent Reference 2] JPA 2003-337293